The benchmark Korea Composite Stock Price Index shed 8.21 points, or 0.35 percent, to close at 2,370.30.
Trade volume was light at 224 million shares worth 3.94 trillion won ($3.53 billion), with gainers outnumbering losers 438 to 361.
The local stock market opened lower as investors took to the sidelines after Federal Reserve Chair Janet Yellen skirted making any comments on monetary matters at a key policy meeting on Friday.
Also, the won's gain versus the greenback prompted concerns that local exporters' earnings growth may slow down.
"Foreign investors are tempted to unload local stocks as the won's strength against the US dollar is feared to dent local exporters' bottom line," said Lee Kyong-min, an analyst at Daishin Securities.
Adding to downward pressure on the local stock market, Samsung Group affiliates lost ground.
Samsung Electronics, the top market cap here, dropped 1.96 percent to end at 2,305,000 won. Last week, the group's heir Lee Jae-yong was sentenced to a jail term of five years for bribery, embezzlement and other charges in a massive corruption scandal that led to the ouster of former President Park Geun-hye.
Lee's legal representatives said they will immediately appeal the case. Prosecutors earlier sought 12 years against the Samsung heir.
Samsung C&T, the de facto holding firm of the group, also shed 3.37 percent to end at 129,000 won.
SK hynix, a major chipmaker, declined 0.73 percent at 67,900 won, and POSCO, the top steelmaker, remained flat at 334,500 won.
Hyundai Motor, South Korea's No. 1 automaker, remained unchanged at 144,000 won, and its smaller affiliate Kia Motors sank 0.43 percent to finish at 34,500 won.
In contrast, LG Chem, the country's No. 1 chemical firm, gained 0.27 percent to end at 144,500 won, and AmorePacific, leading cosmetics maker, rose 1.55 percent, to end at 295,500 won.
The local currency closed at 1,119.90 won against the US dollar, up 8.6 won from the previous session's close.
Bond prices, which move inversely to yields, ended lower. The yield on three-year Treasurys gained 0.2 basis point to 1.758 percent and the return on the benchmark five-year government bond rose 0.3 basis point to 1.963 percent. (Yonhap)